This invention relates generally to monitoring gas turbine engine performance and, more particularly, to a method and system for measuring a lower heating value and Wobbe Index for a gaseous fuel.
A demand is increasing for gas-fired combustion turbines and reciprocating engines that can be reliably operated on Liquefied Natural Gas (LNG) or blends of pipeline natural gas and LNG, as well as a wide variety of low BTU gaseous fuels. Thus, there is need for a rapid and inexpensive method and system to determine a lower heating value (LHV) and/or a Wobbe Index of these fuels in order to ensure that engine performance is properly matched to the fuel characteristics.
Existing methods for determining the LHV and/or the Wobbe Index of a gaseous fuel include the use of a gas chromatograph (GC) system having a glass capillary system to separate the fuel constituents and a thermal conductivity detector (TCD) and/or a flame ionization detector (FID) to quantitatively identify the constituents. However, conventional GC systems have several drawbacks. GC systems are relatively expensive, typically costing about US$20,000.00 or more. Additionally, GC systems are difficult to operate, requiring significant training. Further, GC response is relatively slow, in that analysis times of several minutes are normally required.
Another method for determining the LHV and/or the Wobbe Index of a gaseous fuel is based on a correlation existing between the Wobbe Index and the density and dynamic viscosity of the fuel. Yet another method includes using a calorimeter to obtain a measure of fuel quality. For example, it is possible to exploit the correlation between thermal conductivity of the fuel gas and the heating value. This correlation exists for typical natural gas that is composed mostly of methane. However, problems with measurement accuracy may develop as fuel compositions vary.
The Wobbe Index (W) is a commonly used measurement of fuel quality that is given by the following equation:
                    W        =                  LHV                      SG                                              (                  Eq          .                                          ⁢          1                )            
wherein SG is the specific gravity of the fuel being measured. Using this correlation, a Wobbe Index of a wide range of fuels can be measured. However, there is a drawback to this method. The correlation does not hold if there is a high concentration of some gases, such as H2, CO2 and/or LPG.